1. Field of the Invention
The invention relates to a driving circuit and related method for providing feedback control and open-circuit protection, and more particularly, to a driving circuit and related method utilizing an analysis and decision circuit to detect status of light-emitting devices for providing feedback control and open-circuit protection.
2. Description of the Prior Art
Currently, light emitting diodes (LEDs) are developed and applied to backlight modules for replacing conventional CCFLs since LEDs have advantages of small size and low power consumption. Light emitting diodes have found a myriad of applications in many electrical circuits produced for consumer, commercial, industrial, and military uses. LEDs are semiconductor devices that convert electrical energy directly into light, and, like many other electrical components, are susceptible to damage or destruction when exposed to excessive currents or voltages. When the need arises, circuits can be designed to provide protection to devices that may encounter over-currents and over-voltages. LEDs are often used as light indicators or other light sources for portable electronic devices such as mobile phones, notebook computers, and personal digital assistants (PDAs). However, there have been increasing demands for LEDs to be applied to larger displays such as large neon signs. Such applications require many LEDs for producing a sufficient amount of light. Since the forward-biased current of an LED increases exponentially with the applied forward-biased voltage, it is desirable to drive LEDs with current sources to achieve matching luminance of different LEDs.
Please refer to FIG. 1. FIG. 1 is a diagram of a conventional driving circuit 100 in the prior art. The driving circuit 100 includes a voltage regulator circuit 110, a light-emitting device 120, and a constant-current supplier 130. The voltage regulator circuit 110 has a first input end 112 for receiving an input voltage VIN, a second input end 114 for receiving a feedback signal FB, and an output end 116 coupled to an input end of the light-emitting device 120. The voltage regulator circuit 110 is used for providing a driving voltage VDD to the light-emitting device 120. The constant-current supplier 130 provides a constant current Ic for driving the light-emitting device 120. In otherwords, the constant-current supplier 130 can dynamically adjust an amount of luminance of the light-emitting device 120 according to adjustments in the current value of the constant current Ic.
As shown in FIG. 1, the light-emitting device 120 includes a plurality of light emitting diodes 140. Note that each light emitting diode 140 is referred to as a current driven device. Also, the luminance of the light emitting diode is proportional to the constant current Ic. That is, the luminance of each light emitting diode 140 increases as the constant current Ic increases. In general, to achieve a uniform luminance in the plurality of light emitting diodes 140 it is a matter of driving each current of the plurality of light emitting diodes 140 with a same current. To achieve the requirement of uniform luminance, the light emitting diodes 140 will be coupled in a series. That is, as more light emitting diodes 140 are coupled, a required forward bias voltage Vf of the light-emitting device 120 grows. Therefore, the voltage regulator circuit 110 must provide more driving voltage VDD to supply the required forward bias voltage Vf of the light-emitting device 120.
Please refer to FIG. 2. FIG. 2 is a diagram of a conventional driving circuit 200 in the prior art. The driving circuit 200 includes a voltage regulator circuit 210, six light-emitting devices 221-226, six constant-current suppliers 231-236, and a select circuit 250. The driving circuit 200 is similar to the driving circuit 100 in FIG. 1. The difference between them is that the driving circuit 200 is coupled to more light-emitting devices and further includes the select circuit 250. In this embodiment, there are six light-emitting devices 221-226, but can be expanded to even more or even less light-emitting devices. The voltage regulator circuit 210 is used for providing a driving voltage VDD to the six light-emitting device 221 -226. The first constant-current supplier 231 provides a first constant current 11 for driving the first light-emitting device 221. To reason by analogy, the sixth constant-current supplier 236 provides a sixth constant current I6 for driving the sixth light-emitting device 226.
However, due to limitations of the materials and the manufacturing process used for LEDs, the required forward bias voltage of each light emitting diode 240 is not identical. For example, consider that the first light-emitting device 221 includes three light emitting diodes 240. As is known, the required forward bias voltage of each light emitting diode 240 is not identical. Therefore, the six light-emitting devices 221-226 will have different forward bias voltages Vf1-Vf6. In this embodiment, the driving circuit 200 utilizes the select circuit 250 to select the smallest of six voltage levels Vdrop1-Vdrop6 to output a minimum voltage level VN to an input end 214 of the voltage regulator circuit 210. That is, to reduce the power consumption for each constant current supplier 231-236, and to ensure that all the six light-emitting device 221-226 can operate smoothly, the select circuit 250 thus selects the smallest of the six voltage levels Vdrop1-Vdrop6 to be a minimum voltage level VN (which are respectively corresponding to the biggest forward bias voltage of the voltage level Vf1-Vf6) to output the feedback signal FB.
Please refer to FIG. 1 and FIG. 2, the driving voltage VDD can be adjusted according to the feedback signal FB. If the driving circuit needs to drive a plurality of light-emitting devices, the select circuit 250 can select the smallest voltage levels Vdrop1-Vdrop6 to output a minimum voltage level VN to be the feedback signal FB. That is, to reduce the power consumption for each constant current supplier 231-236. Assuming that the second light-emitting device 222 is burned out (or an open-circuit), the select circuit 250 will always select the voltage level Vdrop2 to be the feedback signal FB. Under this condition, the driving voltage VDD keeps raising all the time. If the driving voltage VDD is greater than a maximum value that the driving circuit 200 can bear, the whole driving circuit or its elements may become damaged.